1. Field of the Invention
The present invention relates to a magnetic disk apparatus having magnetic disks and magnetic heads provided within its enclosure, and, more particularly, to a magnetic disk apparatus having excellent shock resistance to an externally-applied shock.
2. Description of the Related Art
A magnetic disk apparatus has rotatable magnetic disks and magnetic heads for writing and reading information on and from the magnetic disks. Each magnetic head is positioned over a desired track position of the associated magnetic disk to read data from that track or write data thereon.
This magnetic disk apparatus is widely used as an external storage device. Recent demand tends toward downsizing of computer systems using such a magnetic disk apparatus. Particularly, the use of such a magnetic disk apparatus in a notebook type computer, a word processor, a portable, palmtop computer and so forth has become popular. There is therefore a demand for small and slim magnetic disk apparatuses, such as a 1.8-inch magnetic disk apparatus. Micro magnetic disk apparatuses with a size of 1.8 inches or less have the size of a credit card, This type of magnetic disk apparatus can be detachably mounted in a computer and can be handled alone.
When this magnetic disk apparatus is handled alone unlike the one installed in a computer, it is susceptible to an external magnetic field and external electric noise, which may cause read/write errors. Similarly, if the detachable magnetic disk apparatus is dropped on a floor or the like by accident, it would receive a great shock, which could damage the internal components.
Such a magnetic disk apparatus therefore requires good shock resistances resistance to external magnetic fields and resistance to electric noise. To reduce an external shock on the magnetic disk apparatus it has been proposed to provide a shock absorber near the magnetic heads to prevent the head from being damaged by a shock (see Japanese Unexamined Patent Publication No. 168985/1991).
To protect magnetic disks from the internal magnetic field generated by the actuator in the magnetic disk apparatus, it has been proposed that the base on which the actuator is mounted should be made of a ferromagnetic material (see Japanese Unexamined Patent Publication No. 63077/1984).
However, the conventional shock absorbing scheme for magnetic disk apparatuses probably raises the following problem. In a small magnetic disk apparatus, the shock resistance can be improved without the use of the aforementioned shock absorbers by designing the magnetic heads smaller to reduce the shock mass. In other words, even if the magnetic disk apparatus receives a shock, the magnetic heads or magnetic disks can be prevented from being damaged by the vibration of the magnetic heads due to the lighter mass of each magnetic head (e.g., 1 g or below).
The mass of the magnetic disk, however small it is made, is larger than the mass of the magnetic head. In a slim magnetic disk apparatus, the clearance between the magnetic disks and the housing of the magnetic disk apparatus is small. So is the gap between the magnetic disks. When magnetic disks in a slim magnetic disk apparatus vibrate by an external shock applied thereto, therefore, the magnetic disks may hit against the inner wall of the housing of the magnetic disk apparatus or hit against one another, and may thus be damaged. This can disable data recording/reproduction on the damaged disks. Further, when the magnetic disks vibrate, their magnetic heads may hit against the housing so that the magnetic disks and the magnetic heads may both be damaged.
The aforementioned proposal of providing shock absorbers is a measure to cope with an external shock in the case where a magnetic disk apparatus is installed in a computer or the like, the magnitude of the shock is about 100 G to 200 G. However, in a possible case where the magnetic disk apparatus is dropped on a floor or the like, the magnitude of the shock exceeds 200 G and rises above 300 G. Accordingly, the magnetic disks securely attached to the spindle motor vibrate and protected by the conventional shock absorbers, possibly causing a likely damage to the magnetic disks, the magnetic heads, etc.